The present invention relates to an improved thermosensitive recording material, and more particularly to a thermosensitive recording material comprising a support material and a thermosensitive color-forming layer formed on the support material, which thermosensitive color-forming layer comprises a leuco dye which is a colorless or light-colored at room temperature and a color developer capable of inducing color formation in the leuco dye upon application of heat thereto.
Recently thermosensitive recording materials are employed in a variety of fields, for instance, for use with printers of computers, recorders of medical analytical instruments, facsimile apparatus, automatic ticket vending apparatus, and thermosensitive copying apparatus, since they have the following advantages over other recording materials: (1) Images can be formed by simple heat application, without any complicated steps for development; (2) the thermosensitive recording materials can be produced by a simple apparatus and the storage of the thermosensitive recording materials is simple and does not involve excessive costs; (3) as the support material of the thermosensitive recording materials, paper is usually used, which is rather inexpensive in comparison with other support materials, such as synthetic resin films, and when paper is used as the support material, the thermosensitive recording material has a pleasing plain-paper-like touch.
A conventional thermosensitive recording material is produced by coating a support material (for instance, a sheet of paper or a synthetic resin film) with a thermosensitive color-forming layer formation liquid containing a color-forming component and a color developing component which reacts with the color-forming component to form color when heated, and then by drying the liquid to form a thermosensitive color-forming layer.
Images are formed and recorded in the thus produced thermosensitive recording material by application of heat through a thermal pen or head.
Thermosensitive recording materials of the above-described type are disclosed, for instance, in Japanese Patent Publications No. 43-4160 and No. 45-14039.
The conventional thermosensitive recording materials have the shortcomings that they are slow in thermal response, not allowing rapid recording with high image density and high image sharpness.
In order to increase the thermal coloring sensitivity of these thermosensitive recording materials, there have been proposed methods in which a particular thermo-fusible material is added to the thermosensitive color-forming layer, thereby attaining high thermal coloring sensitivity and allowing rapid recording with high image density and high image sharpness.
Examples of such thermo-fusible materials are disclosed, for instance, in the following Japanese laid-open patent applications: nitrogen-containing compounds, such as acetamide, stearamide, m-nitroaniline, and phthalic acid dinitrile in Japanese Laid-Open Patent Application No. 49-38424; acetoacetanilide in Japanese Laid-Open Patent Application No. 52-106746; N,N-diphenylamine derivatives, benzamide derivatives and carbazole derivatives in Japanese Laid-Open Patent Application No. 53-11036; alkylated biphenyls and biphenyl alkanes in Japanese Laid-Open Patent Application No. 53-39139. In Japanese Laid-Open Patent Application No. 56-144193, there are disclosed p-hydroxybenzoic acid ester derivatives which serve as thermo-fusible materials and as color developing materials.
Of the above compounds, p-hydroxy benzoic acid ester derivatives have been considered to be the best to be used as color developer in the thermosensitive color-forming layer of the thermosensitive recording materials.
However, the method of using p-hydroxybenzoic acid derivatives still has the shortcoming that the recorded images fade and white powder or a crystal-like material appears on the surface of the image portions of the thermosensitive recording materials, so that the image portions are whitened.